1. Field of the Invention
The present invention relates to an apparatus for measuring the acoustic anisotropy of materials, and particularly to an apparatus for determining the acoustic anisotropy of a material while under increased pressure and temperature.
2. Discussion of the Related Art
In the oil and gas industry, once a well has been drilled and shows the presence of oil or gas, a determination is made on whether it would be economical to produce. A primary factor in determining whether a well may be economical is the flow rate of the product from the formation into the well. For marginally economic wells, the flow rate may be enhanced by fracturing the reservoir formation to expose more surface area of the formation to the well bore. The orientation of induced fractures in a formation may depend upon the amount of overburden, the formation's principal stress axes, bedding planes, formation homogeneity or cementation. In some cases the principal stress axes control the orientation of fractures.
Generally, fracture planes induced in the formation occur in the plane of the two greatest principal stress axes. For example, if the greatest principal stress axis of a formation is along the vertical plane, fractures induced in the formation most likely will occur in some vertically oriented plane. Orientation of in-situ principal stress axes can sometimes be determined by measuring the velocity anisotropy of samples taken from the subsurface interval of interest. The technique is based on the theory that an acoustic pulse propagates through a homogenous material of unit dimension at the same velocity along all axes. If the stress is greater along one of the other axes and then relieved, microfractures will sometimes develop in planes substantially perpendicular to that stress axis. The presence of microfractures generally decreases the effective velocity of acoustic waves propagating through the material. Thus it has been found that acoustic velocity can be lower along the axis of greatest stress.
Traditionally the engineer would determine the acoustic anisotropy of a formation by extracting an oriented core sample from the reservoir formation. At least two additional smaller samples were taken from the larger core along different axes. Each small sample, having the same dimensions, would be placed between two transducers. One transducer would generate an acoustic signal in the sample. This pulse would travel through the core and be detected by the other transducer at the opposite end. The differences in the acoustic wave travel time through the two samples can reveal the orientation of past stress axes. A major disadvantage with the traditional technique is that the samples were measured at ambient pressure and temperature which provides incomplete data sets. Additionally, a separate sample was required for a minimum of three measurement directions, thus requiring an inordinate amount of time and expense.
There has been a long felt, yet unsolved need, for a method and apparatus for measuring the acoustic anisotropy of a single sample. Moreover there has been a long felt need for a method and apparatus for measuring the acoustic velocity of a sample under conditions substantially similar to those experienced by the sample in nature.